1. Field of the Invention
This invention relates to methods and compositions for cleaning metal surfaces, and more particularly, to methods and compositions for removing elemental copper and copper oxide deposits from metal surfaces.
2. Description of the Prior Art
The operation of process equipment such as steam boilers, heat exchangers, feed water heaters and other equipment through which water is circulated is often hindered by the formation of water insoluble deposits on the interior metal surfaces thereof. Such deposits often contain various forms of iron such as iron salts and iron oxides, e.g., magnetite and hematite, as well as copper in the form of elemental copper and copper oxides. The presence of water insoluble deposits on the interior metal surfaces of process equipment can decrease the capacity of flow passages, interfere with proper heat transfer and lead to leaks and ruptures which necessitate undesirable down time and maintenance costs.
In order to prevent the above problems from occurring, a variety of methods and solvents have been developed for removing water insoluble deposits from the interior metal surfaces of equipment. The type of method and solvent employed depends primarily on the nature of the deposits involved. Typical solvents include acids such as hydrochloric acid and nitric acid, and ammonia or amine salts of organic chelating acids such as citric acid and ethylenediaminetetraacetic acid (EDTA). Many methods and solvents are designed for the removal of both iron and copper containing deposits. For example, in U.S. Pat. No. 3,248,269 to Bell, a two step process for removing both copper and iron deposits from metal surfaces is disclosed. First, the surfaces are contacted with a neutral ammonium citrate solution to dissolve iron oxides. During the course of this reaction, ammonia and/or ammonium hydroxide is produced in situ which raises the pH of the solution. The ammoniacal solution together with iron and/or iron salts, e.g., ferrous citrate, formed in the first step then dissolve copper oxides.
Unfortunately, the number of methods and solvents available for dissolving only elemental copper and copper oxides from metal surfaces is limited. Methods and solvents such as the method and solvent described above designed for dissolving both iron and copper deposits typically do not effectively remove copper deposits by themselves, i.e., such methods and solvents only effectively remove copper deposits after they have been used to dissolve iron deposits. When iron deposits are not involved, copper deposits are most commonly removed by ammoniacal solvents containing oxidizing agents. The oxidizing agents oxidize elemental copper to copper oxide while ammonia or ammoniacal compounds dissolve copper oxide. Conventional oxidizing agents employed in these solvents include sodium bromate [NaBrO.sub.3 ] and ammonium persulfate [(NH.sub.4).sub.2 S.sub.2 O.sub.8 ]. Sodium bromate is the most widely used.
While ammoniacal solvents employing sodium bromate or ammonium persulfate as an oxidizing agent effectively remove elemental copper and copper oxide deposits in the absence of iron, they are very hazardous to use and difficult to dispose of. Sodium bromate decomposes upon contact with acid yielding bromine, a poisonous gas. Inasmuch as copper removal processes are often performed in conjunction with acid cleaning, the potential for bromine generation commonly exists. Both sodium bromate and ammonium persulfate are strong oxidizing agents. As a result, the potential for fires and explosions when handling these oxidizing agents is high. If solutions of sodium bromate and/or ammonium persulfate impregnate combustible material such as wood, paper or clothing and are allowed to dry, impact or friction can cause the material to ignite.
In order to dispose of solvents containing sodium bromate and/or ammonium persulfate, the sodium bromate and/or ammonium persulfate must be neutralized or reacted with a reducing agent. This results in further personnel hazards, extra storage and mixing requirements and additional expense.
By the present invention, a process for safely removing elemental copper and copper oxide deposits from metal surfaces without first removing iron containing deposits therefrom is provided.